1. Field of the Invention
The present invention relates to a piston ring made with a ceramic material for an internal combustion engine.
2. Description of the Prior Art
Piston rings consisting of cast iron or steel material which are often chromium-plated or have a hard metal coating, e.g., of molybdenum, on their rolling surfaces are known. German Offenlegungsschrift No. 14 26 120 discloses molybdenum or tantalum for use in rolling track coating of piston rings. This Offenlegungsschrift also discloses a piston ring coating of high hardness metal compounds, e.g., carbides, oxides, nitrides, borides, silicides and others. German Offenlegungsschrift No. 19 50 081 discloses grooves cut on the outer wall of an oil scraper ring, into which metal oxides or metal carbides are introduced by spray deposition or metallization. Other proposals, such as, in DE-AS No. 26 20 104, DE-OS Nos. 29 26 879, 29 34 027, 29 38 151, 29 42 037, 30 17 907 and 31 02 221, provide for metal carbide coatings or coatings of metal/metal carbide-mixtures, which are applied on the rolling surfaces of piston rings, e.g., by means of plasma jet application.
These suggestions all have a common disadvantage which resides in the problems inherent in the ceramic/metal bond, namely, that materials of different thermal expansion have a different degree of expansion under thermal stress and, as a result, there is the risk that the applied layers may chip off. This danger exists especially for piston rings which are coated with poorly conducting materials. Such piston rings cannot remove the heat generated in the combustion process to the cylinder walls. Additionally, piston rings coated only on their rolling surface may form a phenomenon similar to welding in the piston ring groove between the material of the piston and the metal of the piston ring due to the great heat and thus impair the mobility of the piston ring. Another disadvantage of the coated piston rings lies in the complicated design of the layers and of the manufacturing processes required to make such rings. The relatively high degrees of porosity required, i.e., a porosity up to 20%, in DE-OS 29 42 037, and up to 30% in DE-OS 29 38 151, is also disadvantageous. These porous layers have relatively low strengths and are subject to high wear.